A lens system disclosed by Japanese Patent Kokai Publication No. Hei 2-176612 is known as a conventional example of compact objective lens system for endoscopes. This conventional example has a composition shown in FIG. 27, wherein the objective lens system is composed, in order from the object side, of a first lens component L.sub.1 which is a plane parallel cover glass plate, an aperture stop S, a second plano-convex lens component L.sub.2 and a third plano-convex lens component L.sub.3.
A lens system disclosed by U.S. Pat. No. 4,858,002 is known as another conventional example. This conventional example has a composition shown in FIG. 28, wherein the lens system is composed, in order from the object side, of an aperture stop S, a first meniscus lens component L.sub.1 and a second plano-convex or meniscus lens component L.sub.2.
Furthermore, a lens system disclosed by Japanese Patent Kokai Publication No. Sho 64-33516 is known as still another conventional example. This conventional example has a composition which is similar to that of the lens system disclosed by Japanese Patent Kokai Publication No. Hei 2-176612 and comprises an infrared absorption filter (color correction filter) disposed after the lens system as shown in FIG. 29. Speaking concretely, the lens system is composed, in order from the object side, of a first lens component L.sub.1 which is a cover glass plate, an aperture stop S, a second lens component L.sub.2, a third lens component L.sub.3 and an infrared absorption filter F.
A CCD which is used for electronic endoscopes and the like has sensitivity in an infrared region. It is therefore necessary to insert an infrared absorption filter into an objective lens system for electronic endoscopes which uses a CCD in order to enhance color reproducibility.
The Japanese Patent Kokai Publication No. Hei 2-176612 and U.S. Pat. No. 4,858,002 disclose lens systems which use CCDs as image pickup means, but make no reference to an infrared absorption filter.
Furthermore, Japanese Patent Kokai Publication No. Sho 64-33516 discloses a lens system in which an infrared absorption filter F is disposed, but provide no description of a condition required for the lens system to dispose the infrared absorption filter effectively in a remarkably restricted space and freedom. Speaking more concretely, the infrared absorption filter F is disposed between the third lens component L.sub.3 and the CCD as shown in FIG. 29 but the third lens component L.sub.3 cannot be worked in this case since it has a small marginal thickness. Though no restriction is imposed on the lens system in a case where the infrared absorption filter before the aperture stop, a cover glass plate which is to be disposed at this location will have a large outside diameter in this case. Furthermore, optical path lengths in the filter are different between a central portion and a marginal portion of an image, thereby being apt to produce color ununiformity.
It is general to use an infrared absorption filter having a thickness of 0.5 mm to 2 mm, which remains unchanged even for a compact CCD. Accordingly, a space for the infrared absorption filter is narrower as a lens system uses a CCD which is more compact. In order to prevent flare from being produced by an infrared semiconductor laser beam which has a high output for medical treatment, for example, an infrared absorption filter which is an thick as possible is preferable, thereby making it necessary to reserve a large space.
Furthermore, the conventional examples disclosed by Japanese Patent Kokai Publication No. Hei 2-176612 and U.S. Pat. No. 4,858,002 have field angles which are not so large and these publications provide no description of any means to correct lateral chromatic aberration which is problematic in objective lens systems having remarkably large field angles exceeding 120.degree..
The present invention has an object to provide an objective lens system which is not enlarged by disposing an infrared absorption filter required for electronic endoscopes, has a remarkably large field angle and favorably corrects lateral chromatic aberration.